The present invention relates to a semiconductor integrated circuit device and more particularly to that including a Class B push-pull circuit.
In the prior art, a Class B push-pull circuit using two power output transistors TR1 and TR2 as shown in FIG. 1 is well known. The collector-emitter paths of the transistors TR1 and TR2 are connected in series between a power source Vcc and the ground. The base of the transistor TR1 is connected through a resistor R1 with the emitter of a driver transistor TR3 of which the base is coupled with a signal input and the collector is coupled with the power source Vcc. The base of the transistor TR2 is coupled through a resistor R2 with the collector of a driver transistor TR4 of which the emitter is coupled with the collector of the transistor TR2 and the base is coupled with the signal input terminal 1. Diodes D1 and D2 are used to compensate for the variation in operation characteristics due to temperature change of the transistors TR3 and TR4.
The collector of the transistor TR2 is connected through a resistor R3 to the emitter of the power transistor TR1 and grounded through a coupling capacitor C and a load resistor R0. The emitter of the power transistor TR2 is grounded through a resistor R4. The output signal is produced from an output terminal 2 coupled with the collector of the transistor TR2.
When a power switch (not shown) is turned on and the power source Vcc is electrically coupled with the push-pull circuit, current for charging the coupling capacitor C flows through the collector-emitter path of the power transistor TR1. FIG. 2 shows the relation of the current Ic flowing into the transistor TR1 and the voltage V.sub.CE across the collector-emitter path of the transistor TR1. In the figure, a point A indicates the current Ic and the voltage V.sub.CE of the transistor TR1 at the stationary state, when no signal is supplied thereto. In the worst condition, the voltage Vcc is directly applied to the transistor TR1 and thus a large current flows therethrough, resulting possibly in destruction of the transistor TR1. On the other hand, the transistor TR2 does not operate until the capacitor C is completely charged. For this, the transistor TR2 will operate in the load operation mode. Accordingly, the transistor TR2 is hard to be destroyed compared with the transistor TR1. In the integrated circuit, a high voltage power source is used so that there is a high possibility that the transistor TR1 is destroyed. Particularly in bipolar transistors, the area of safe operation where the transistor is safely operable is smaller as the operation voltage increases. Therefore, the transistor TR1 used under such an unfavorable condition is more apt to be destroyed.